Filtration units, for example of the single-use type, are known which have the appearance of a body constituted by two parts or shells, between which a filtration element is interposed, such parts or shells being mutually assembled and sealed.
The filtration element can have various shapes, for example a pleated flat or circular shape or another shape, depending on the specific constructive requirements.
As mentioned, the body is provided by means of two shells, which are obtained by molding thermoplastic material: a first shell, which is constituted for example by a first tubular connector, for example of the female Luer-Lok type, in which there is an opening for the passage of a fluid and which expands radially into a first annular flange, and a second shell, which is constituted by a second tubular connector, for example of the male Luer-Lok type, in which there is an opening for the passage of a fluid and which expands radially into a second annular flange, or having other shapes.
The two tubular connectors are designed to be connected for example to the tubes of a biomedical line for hemodialysis or other purposes.
The fluid to be filtered, by passing through the filtration unit, necessarily has to pass between the dense meshes of the membrane-based filtration element and thus be purified of the coarse impurities.
The first and second shells are in fact assembled so that the respective flanges face each other and so that the membrane-based filtration element is interposed between them, providing a forced passage for the fluid.
The main techniques currently in use to anchor the filtration element to the two shells provide for gluing it to each of said shells by means of hot-melt polymeric adhesives or polyurethane resins, which may be centrifuged beforehand. Another of the techniques currently in use provides for welding the filtration element to each shell by means of ultrasound or by heating, with heating lamps or other radiating bodies, a portion of the shell with subsequent embedding of the membrane-based filtration element in the portion of the shell thus heated and rendered partially fluid.
Once the filtration element has been fixed to the two shells, they are mutually assembled and sealed with various known methods, which include, for example, welding by hot-melt adhesive, ultrasound welding, gluing and overmolding thermoplastic material at their joining line.
However, these methods for providing known types of filtration unit and in particular these techniques for fixing the filtration element to the shells of the filtration body are not free from drawbacks, which include the fact that the use of hot-melt adhesives or polyurethane resins to fix the membrane-based filtration element to the shells does not ensure a sufficient mechanical strength and heat resistance of the joint, which is therefore subject to rapid and unpredictable deterioration.
Techniques for fixing the filtration element to the shells of the body of the filtration unit which use ultrasound welding or heating of the support by means of lamps or other radiating bodies also are not free from drawbacks; to perform these techniques, although they still ensure a high mechanical strength and heat resistance of the joint between the filtration element and the shell and therefore of the filtration unit, however, long heating times are required.
Further, another drawback observed in these last techniques for fixing the filtration element to the shell is linked to the excessive costs entailed for apparatuses for the targeted heating of a portion of the shell chosen for the fixing of the filtration element.
Another drawback in these known techniques is that shell size variations occur, therefore leading to the possibility of forming thermal stresses therein due to heating after the step for forming the shell.